0. CCNA Exam Questions
9 InterVLAN Routing
9.2 InterVLAN Routing
1. 9.2.3 InterVLAN 3 - Router-on-a-stick
  1. 9.2.3.x VLAN 1 doesn’t require setup cuz it’s the default VLAN
2. 9.3 Static Routing Entries
10 Dynamic Routing

7th Lesson - CCNA Fast Track (June 03, 2025). We left off at page 124.

0. CCNA Exam Questions

MC will cover Routing Entries Matching orders, 2 Questions in the past, study this topic well.
- Distinguish between what is route and what is non-route
  - Those with eng characters are routes lmao
  - We can count up the routes, in the example of pg.137 we have 7 routes in total. We include static route and default route !
On the exam we have DR and BDR election stuff, every other device is priority value = 0
Changing OSPF priority value on CCNA exam

9 InterVLAN Routing

9.2 InterVLAN Routing

9.2.3 InterVLAN 3 - Router-on-a-stick

High level understanding:
- Router-on-a-stick = a trunk link (between switch and router) carrying all sub-routed VLAN traffic!
- Achieved by allowing the an interface port to get 802.1Q trunk, thus forming trunk link
- For packets to leave a VLAN group, must do it through the default gateway
Pros and cons of Router-on-a-stick
- Pro:
  - Suitable for classical equipments
  - No need multi-layer/L3 involvement, L2 is also okay
  - This method for sub-interfacing is over 20 years old lmao, very backwards compatiable
  - Non-cisco protocol
- Cons:
  - SPOF = Single point of failure; Due to us having only 1 trunk link
  - Data packet Traffic in trunk link
Q: How to turn on sub-interfaces?
A: You need 2 things:
- set VLAN
- set IP

9.2.3.x VLAN 1 doesn’t require setup cuz it’s the default VLAN

Current Topology:

Step 1: Setup VLAN 2 on switch 1 (no need to setup VLAN 1)

Note: no vlan 1 is impossible lmao
Note: switchport mode access means this port is NEVER trunk link
Note: switchport trunk encapsulation dot1q must come before switching to trunk mode!

  en
  config t

  vlan 2

  int g1/1
  switchport mode access
  switchport access vlan 1

  int g0/2
  switchport mode access
  switchport access vlan 2

  int g0/0
  switchport trunk encapsulation dot1q
  switchport mode trunk

  end

Sanity check with do sh vlan briefor sh vlan brief
Step 2: Set up Router 3 (configure 802.1q trunk interface for VLAN 1 & VLAN 2)
- Note: encapsulation dot1q 1 native is considered to be the sub-interfacing step and it’s connected to VLAN 1
- In VLAN 2, we don’t put native VLAN obviously
- We don’t put no shut for both VLAN 1 and VLAN 2, as we assume the main interface is no shut by default, others should be too
```
  en
  config t
  hostname Router3

  int g0/0
  no ip address
  no shut

  int g0/0.1
  encapsulation dot1q 1 native
  ip address 10.0.0.1 255.0.0.0

  int g0/0.2
  encapsulation dot1q 2
  ip address 172.16.0.1 255.255.0.0

  end
```
- no shut = Physical interfaces for g0/0 needs to be no shut
- no ip address = If there’s any IP set on the physical interface, then IP needs to be removed with this
- int g0/0.1 = This creates sub-interface 1, and will enter sub-interface Config mode (“subif”)
  - Note: By convention, we set the VLAN ID the same as our sub-interface #, it’s better trust me.
- encapsulation dot1q 1 native = native means native VLAN, all frames sent here has NO VLAN tag!
- encapsulation dot1q 2 = VLAN 2 is not the native VLAN, it’s responsible for VLAN ID 2
- ip address 10.0.0.1 255.0.0.0 = configuring for the sub-interface
Step 3: Verify on Router 3 using sh ip route

      Codes:  I - IGRP derived, R - RIP derived, O - OSPF derived
              C - connected, S - static, E - EGP derived, B - BGP derived
              * - candidate default route, IA - OSPF inter area route
              E1 - OSPF external type 1 route, E2 - OSPF external type 2 route

      Gateway of last resort is not set
          10.0.0.0/8 is variably subnetted, 2 subnet, 2 masks
      C   10.0.0.0/8 is directly connected, GigabitEthernet1/1
      L   10.0.0.1/32 is directly connected, GigabitEthernet1/1

          172.16.0.0/16 is variably subnetted, 2 subnet, 2 masks
      C   172.16.0.0/16 is directly connected, GigabitEthernet0/2
      L   172.16.0.1/32 is directly connected, GigabitEthernet0/2

Note: LOOKS GOOD
Step 4: Setup Router 1 as well. Note: 0.0.0.0 0.0.0.0 is the default gateway

      en 
      config t
      hostname Router1

      int g0/0
      ip address 10.0.0.100 255.0.0.0
      no shut
      exit

      ip route 0.0.0.0 0.0.0.0 10.0.0.1
      end

Step 5: Setup Router 2 as well. Note: 0.0.0.0 0.0.0.0 is the default gateway

      en 
      config t
      hostname Router2

      int g0/2
      ip address 176.16.0.200 255.255.0.0
      no shut
      exit

      ip route 0.0.0.0 0.0.0.0 172.16.0.1
      end

Step 6: Let’s ping for sanity checks ! Please ping it from Router 2
```
  ping 10.0.0.100
```
Looking good! InterVLAN setup is up and running

  Sending 5, 100-byte ICMP Echos to 10.0.0.100, timeout is 2 seconds:
  !!!!!
  Success rate is 100% (5/5), round-trip min/avg/max = 3/4/6 ms

9.3 Static Routing Entries

9.3.1 Static Routing Introduction

Intro:

Routing table contains only directly connected networks. Beyond that direct interface, it’s not visable
If we refer to this topology:

Let’s look at Router 1 and Router 2’s Routing table before any human intervention

Router 1 routing table:

  Network Destination         Gateway                 Interface
  ---------------------------------------------------------------
  192.168.1.0/24              Directly Connected      s0/0/0
  10.0.0.0/8                  Directly Connected      g0/0

Router 2 routing table:

  Network Destination         Gateway                 Interface
  ---------------------------------------------------------------
  192.168.1.0/24              Directly Connected      s0/0/0
  172.16.0.0/16               Directly Connected      g0/1

Issue: For networks that are NOT directly connected, and admin can manually add the routing entries ! For example…

Router 1 routing table:

  Network Destination         Gateway                 Interface
  ---------------------------------------------------------------
  192.168.1.0/24              Directly Connected      s0/0/0
  10.0.0.0/8                  Directly Connected      g0/0
  172.16.0.0                  192.168.1.2             s0/0/0

Router 2 routing table:

  Network Destination         Gateway                 Interface
  ---------------------------------------------------------------
  192.168.1.0/24              Directly Connected      s0/0/0
  172.16.0.0/16               Directly Connected      g0/1
  10.0.0.0/8                  192.168.1.1             s0/0/0

9.3.2 Configuring static route on Router1 (will setup host route too)

Setup: Will include host route!
Our topology:

Step 1: Setup Router 1

  en
  config t
  hostname Router1

  int g0/1
  ip address 10.0.0.1 255.0.0.0
  no shut

  int g0/0
  ip address 192.168.1.1 255.255.255.0
  no shut

  end

Step 2: Setup Router 2

en
config t
hostname Router2

int g0/0
ip address 192.168.1.2 255.255.255.0
no shut

int g0/1
ip address 172.16.0.2 255.255.0.0
no shut

end

Step 3: Router 1 ping 172.16.0.2, success rate is 0 !
- Can also verify with sh ip route
- Due to: In router 1 doesn’t have 172.16.0.2 in it’s routing table D:
Step 4: Resolve the issue with adding a static route
- Static Route format: ip route <dest network> <subnet mask> then <gateway access> or <outgoing ip interface>. FYI, IPv6 static route configs are similar.
- ```
  config t
  ip route 172.16.0.2 255.255.0.0 192.168.1.2
```
  9.3.2.x Network Route … Static Route :D
  Note: 192.168.1.2 is set as static route here, it’s also known as a network route !

Step 5: Sanity Check on Router 1 with sh ip route

The summary returned

  Codes:  L - local, C - connected, s - static, R - RIP, M - mobile, B - BGP
          D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
          N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
          E1 - OSPF external type 1, E2 - OSPF external type 2
          i - IS-IS, su - IS-IS summary, L1 - IS-IS level 1, L2 - IS-IS level 2
          ia - IS-IS inter area, * - candidate default, U - per-user static route
          o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
          a - application route
          + - replicated route, % - next hop override, p - overrides from pfR

  Gateway of last resort is not set
          10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
  C           10.0.0.0/8 is directly connected, GigabitEthernet0/1
  L           10.0.0.0/32 is directly connected, GigabitEthernet0/1
  S       172.16.0.0/16 [1/0] via 192.168.1.2
          192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
  C           192.168.1.0/24 is directly connected, GigabitEthernet0/1
  L           192.168.1.1/32 is directly connected, GigabitEthernet0/1

Note: “S” meaning a static route for the network 172.16.0.0/16

Step 6: ping 172.16.0.2. The packets are now well received

  Sending 5, 100-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:
  !!!!!
  Success rate is 100% (5/5), round-trip min/avg/max = 1/2/4 ms

Step 7: Removing the static route with conf t & no ip route 172.16.0.0 255.255.0.0 192.168.1.2
Step 8: On Router1, set up another ip route
- ```
  conf t
  ip route 172.16.0.2 255.255.255.255 192.168.1.2
  end
```
- Logic behind the 2 parameters, let’s look at the topology again:
  - 172.16.0.2 since that’s our target destination address (next to Router 2)
  - 255.255.255.255 is the host route, we’re using host route, since there’s only 1 IP in the network 172.16.0.0
  - 192.168.1.2 is the gateway address, it’s the interface on Router1 that is closest shoot out interface from Router1

Step 9: Verify and sanity check:

The summary returned

 Codes:  L - local, C - connected, s - static, R - RIP, M - mobile, B - BGP
         D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
         N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
         E1 - OSPF external type 1, E2 - OSPF external type 2
         i - IS-IS, su - IS-IS summary, L1 - IS-IS level 1, L2 - IS-IS level 2
         ia - IS-IS inter area, * - candidate default, U - per-user static route
         o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
         a - application route
         + - replicated route, % - next hop override, p - overrides from pfR

 Gateway of last resort is not set
         10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
 C           10.0.0.0/8 is directly connected, GigabitEthernet0/1
 L           10.0.0.0/32 is directly connected, GigabitEthernet0/1
         172.16.0.0/32 is subnetted, 1 subnets
 S           172.16.0.2 [1/0] via 192.168.1.2
         192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
 C           192.168.1.0/24 is directly connected, GigabitEthernet0/1
 L           192.168.1.1/32 is directly connected, GigabitEthernet0/1

9.3.2.y : Host Route

Note:
- A static route/ host route (/32) for network 172.16.0.2/32 can now found & now marked with “S” as static. Host Route is used since the route only has 1 IP address here.
- Host Route can only access 1 IP

Step 9: ping the target.

  Sending 5, 100-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:
  !!!!!
  Success rate is 100% (5/5), round-trip min/avg/max = 1/2/4 ms

ping is working now

Alternative:
- ip route 172.16.0.2 255.255.255.255 g0/0
- ip route 172.16.0.2 255.255.255.255 192.168.1.2
- Idea: now Router 1 can go to 172.16.0.2/32 via it’s own Router1’s interface g0/0

9.3.3 Static Default Route

Background:
- If we have many static routes to set up for different networks
- Better to use a single default route (1 gateway for all other networks)
- Please note default route is also known as gateway of last resort
Note: we’re using the same topology as 9.3.1’s static route
- Topology:

Step 1: Configure Router1 on int g0/1 and int g0/0

      en
      conf t
      hostname Route1

      int g0/1
      ip address 10.0.0.1 255.0.0.0.0
      no shut

      ip g0/0
      ip 192.168.1.1 255.255.255.0
      no shut

      end
      copy run start

Step 2: Configure Router2 on int g0/0 and int g0/1

      en
      conf t
      hostname Router2

      int g0/0
      ip 192.168.1.2 255.255.255.0
      no shut

      int g0/1
      ip 172.16.0.0.2 255.255.0.0
      no shut

      end
      copy config run

Step 3: Back to Router 1! Let’s set up that default route!
- ```
  conf t
  ip route 0.0.0.0 0.0.0.0 192.168.1.2
  end
  copy run start
```
- Explanation:
  - the default route is : 0.0.0.0
  - the default gateway in Router 1 is set for int g0/0 on Router1’s side
  - You almost only do this when your routing table is empty

Step 4: Verify Router1’s ip table with sh ip route

The default route will show up as S*
- Where S = static route
- Where * = default route
The return info of sh ip route.

Please note that the default route info is shown here as S*

  Codes:  L - local, C - connected, s - static, R - RIP, M - mobile, B - BGP
          D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
          N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
          E1 - OSPF external type 1, E2 - OSPF external type 2
          i - IS-IS, su - IS-IS summary, L1 - IS-IS level 1, L2 - IS-IS level 2
          ia - IS-IS inter area, * - candidate default, U - per-user static route
          o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
          a - application route
          + - replicated route, % - next hop override, p - overrides from pfR

  Gateway of last resort is 102.168.1.2 to network 0.0.0.0

  Gateway of last resort is not set
  S*      0.0.0.0/0 [1/0] via 192.168.1.2
          10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
  C           10.0.0.0/8 is directly connected, GigabitEthernet0/1
  L           10.0.0.0/32 is directly connected, GigabitEthernet0/1

          192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
  C           192.168.1.0/24 is directly connected, GigabitEthernet0/1
  L           192.168.1.1/32 is directly connected, GigabitEthernet0/1

Step 5: Try to ping the ip on Router 2’s side which is 172.16.0.2 with ping 172.16.0.2

Returned on the console we have:

Packets are well received

  Sending 5, 100-byte ICMP Echos to 172.16.0.2, timeout is 2 seconds:
  !!!!!
  Success rate is 100% (5/5), round-trip min/avg/max = 1/2/4 ms

9.3.4 Routing Entries Matching Order

Background:
- Please note this topic is related to longest prefix match
- Essentially, we are sorting the importance of these 7 routes !

Step 1 : Assume we hae these 7 routes in the ip routing table

  Codes:  L - local, C - connected, s - static, R - RIP, M - mobile, B - BGP
          D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
          N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
          E1 - OSPF external type 1, E2 - OSPF external type 2
          i - IS-IS, su - IS-IS summary, L1 - IS-IS level 1, L2 - IS-IS level 2
          ia - IS-IS inter area, * - candidate default, U - per-user static route
          o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
          a - application route
          + - replicated route, % - next hop override, p - overrides from pfR

  Gateway of last resort is 102.168.1.2 to network 0.0.0.0

  Gateway of last resort is not set
  S*      0.0.0.0/0 [1/0] via 192.168.1.2
          10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
  C           10.0.0.0/8 is directly connected, GigabitEthernet0/1
  L           10.0.0.0/32 is directly connected, GigabitEthernet0/1

          192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks
  C           192.168.1.0/24 is directly connected, GigabitEthernet0/1
  L           192.168.1.1/32 is directly connected, GigabitEthernet0/1

          192.168.16.0/24 is variably subnetted, 2 subnets, 2 masks
  S           192.168.16.0/26 [1/0] via 192.168.1.3
  S           192.168.16.0/27 [1/0] via 192.168.1.2

    S*      0.0.0.0/0 [1/0] via 192.168.1.2
    C           10.0.0.0/8 is directly connected, GigabitEthernet0/1
    L           10.0.0.0/32 is directly connected, GigabitEthernet0/1
    C           192.168.1.0/24 is directly connected, GigabitEthernet0/1
    L           192.168.1.1/32 is directly connected, GigabitEthernet0/1
    S           192.168.16.0/26 [1/0] via 192.168.1.3
    S           192.168.16.0/27 [1/0] via 192.168.1.2

Step 2: Sorting most specific sub-mask to lowest, aka, /32 is the most specific, /1 is the least specific

Let’s sort by / subnet mask’s size. Note the 192 and 10 has no difference in using this sort method

      L           192.168.1.1/32 is directly connected, GigabitEthernet0/1
      L           10.0.0.0/32 is directly connected, GigabitEthernet0/1
      S           192.168.16.0/27 [1/0] via 192.168.1.2
      S           192.168.16.0/26 [1/0] via 192.168.1.3
      C           192.168.1.0/24 is directly connected, GigabitEthernet0/1
      C           10.0.0.0/8 is directly connected, GigabitEthernet0/1
      S*          0.0.0.0/0 [1/0] via 192.168.1.2

A: we would assume our destination address is 192.168.16.3
Step 3a: Finding the longest prefix route by removing certain routes from this list. Assume our designation is 192.168.16.3 please
- the 2 L’s will be dropped as they only go to 1 place (i.e. host route -> one IP). It cannot go to 192.168.16.3.
- Let’s decide whether the 3rd one on th list will stay 192.168.16.0/27 [1/0] via 192.168.1.2
  - Step 1: Find the network ID and broadcast network
    - network id: 192.168.16.0/27 -> 32 - 27 = 5 host bits -> Max range is 00011111 = 16+8+4+2+1 = 31, ergo the broadcast range can go up to that.
    - broadcast network 192.168.16.31/27
    - meaning the full network range is 192.168.16.0 ~ 192.168.16.31
  - Step 2: Conclusion:
    - Meaning 192.168.16.3 is in range!

Step 4a: Feel free to do sanity check with sh ip route 192.168.16.3

  Routing entry for 192.168.16.0/27
      known via "static", distance 1, metric 0
      Routing Descriptor Blocks:
      * 192.168.1.2
          Route metric is 0, traffic share counter is 1

B: we would assume our destination address is 192.168.16.50
Step 3b: Finding the longest prefix route by removing certain routes from this list. Assume our destination is 192.168.16.50 please
- the 2 L’s will be dropped as they only go to 1 place (i.e. host route -> one IP). It cannot go to 192.168.16.50
- The first S will also be dropped, since the mask /27 doesn’t reach until 192.168.16.50
- What about the other S route is 192.168.16.0/26 via 192.168.1.3
- The available range for 192.168.16.0/26 is…
  - 26 network bits, so 32 - 26 = 6 host bits = 32 + 31 = 63!
  - Hence the broadcast IP can go up to 63 -> 192.168.16.63/26
  - So Yes, this connection can reach 192.168.16.50, no issue

Step 4: Let’s verify this last IP with sh ip route 192.168.16.50

  Routing entry for 192.168.16.50
      known via "static", distance 1, metric 0
      Routing Descriptor Blocks:
      * 192.168.1.3
          Route metric is 0, traffic share counter is 1

9.3.g Handling exceptions if destination address is 1.1.1.1

if we check it with sh ip route 1.1.1.1, we’ll get an error for this Network not in table
We need to handle this with static default routes/ default routes
What Franco mentioned:
- We are forced to use the final trump card: default gateway address 0.0.0.0 (exhausting all host bits)

10 Dynamic Routing

10.1 Introduction

Maintaining static routes in a large company with many routers and many networks… IMPOSSIBLE for network admins.
Using dynamic routing = network admin can maintain routing tables for routers by using routing protocol (e.g. RIP, EIGRP, OSPF, etc…) to add/remove routing entries from ip routing tables AUTOMATICALLY
Let’s look at the topology:
- 1. 10.0.0.0/8 attempting to reach non-directly connected network
- 1. 10.0.0.0/8 is about to do so via a directly connected interface (i.e. 192.168.1.1)
- 1. Routing table automatically adds this entry !
- 1. Similarly, 172.16.0.0 is also attempting to connect to 10.0.0.0
- 1. 172.16.0.0 was able to do so via 192.168.1.2’s interface !
- 1. The logic 172.16.0.0/16 via 192.168.1.2 is auto added to the routing table
- 1. So now, on both sides, 10.0.0.0 can properly reach 172.16.0.0
- 1. and 172.16.0.0 can also reach 10.0.0.0

10.1.1 Distance vector vs link state routing protocols = 2 Routing Protocols

1. Distance Vector (e.g. RIP, IGEP or EIGRP, …)
  - Topology:
  - Core idea: The mechanism ot set up routing one-by-one
  - Distance Vector Routing Protocol = neighbor routers will continue routing and learning them into it’s routing table
1. Link state routing protocols (e.g. OSPF, IS-IS, etc…)
  - Topology:
  - Core idea: Mapping out the router map, exchanging topology info and calculate and conclude
  - Each router calculates the best path to every network into the routing table

10.1.2 Hop Counts

Topology:
Hop Count = # of routers the data packet needs to pass through before arriving at destination
Some routing protocols likes shortest hop counts, I’m talking about you RIP!

10.2 AD / Administrative distance & metric

10.2.1 Administrative distance (AD)

Background:
- Please memorize this table !!!!
- Cisco router’s AD = decides which routing protocol will be providing the route
Table for AD routing values on some common protocols

Name of Route Source	Symbol used in Cisco `sh ip route`	Default AD Value
Connected interface (direct)	C	0
Static Route	S	1
External BGP = eBGP	B (will be discussed in CCNP)	20
Internal EIGRP	D	90
OSPF	O	110
IS-IS	i	115
RIP	R	120
External EIGRP	D EX	170
Internal BGP = iBGP	B (will be discussed in CCNP)	200

How to make use of this table ?
- The network with the lowest AD value will enter the routing table and used for packet routing!

Let’s look at this idea in action:

Topology:
Due to the smallest AD distance, OSPF will triumph over RIP
The summary would be:

  Codes:  L - local, C - connected, s - static, R - RIP, M - mobile, B - BGP
          D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
          N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
          E1 - OSPF external type 1, E2 - OSPF external type 2
          i - IS-IS, su - IS-IS summary, L1 - IS-IS level 1, L2 - IS-IS level 2
          ia - IS-IS inter area, * - candidate default, U - per-user static route
          o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
          a - application route
          + - replicated route, % - next hop override, p - overrides from pfR

  Gateway of last resort is not set

  O       10.0.0.0/0 [110/0] via 192.168.13.3, 00:01:16, GigabitEthernet0/2
          192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
  C           192.168.12.0/24 is directly connected, GigabitEthernet0/2
  L           192.168.12.1/32 is directly connected, GigabitEthernet0/2

          192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
  C           192.168.13.0/24 is directly connected, GigabitEthernet0/2
  L           192.168.13.1/24 is directly connected, GigabitEthernet0/2

As you can see, [110] is referring to OSPF’s AD score and it’s used here !

### 10.2.2 Metric

Background:

If routes are in the same AD, then we determine using the lowest Metric value will enter into the routing table

Note: different routing protocols has different ways of calculating Metrics!

i.e. RIP metric = hop count
Topology:

  Codes:  L - local, C - connected, s - static, R - RIP, M - mobile, B - BGP
          D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
          N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
          E1 - OSPF external type 1, E2 - OSPF external type 2
          i - IS-IS, su - IS-IS summary, L1 - IS-IS level 1, L2 - IS-IS level 2
          ia - IS-IS inter area, * - candidate default, U - per-user static route
          o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
          a - application route
          + - replicated route, % - next hop override, p - overrides from pfR

  Gateway of last resort is not set

  O       10.0.0.0/0 [120/0] via 192.168.13.3, 00:01:16, GigabitEthernet0/2
          192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
  C           192.168.12.0/24 is directly connected, GigabitEthernet0/2
  L           192.168.12.1/32 is directly connected, GigabitEthernet0/2

          192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
  C           192.168.13.0/24 is directly connected, GigabitEthernet0/2
  L           192.168.13.1/24 is directly connected, GigabitEthernet0/2

So we see the metric is [120] means the one with the lowest hop count is chosen :D

Q: What happens if it has the same metric, same AD ?

A; then the packet flow will use each route alternatively. Sharing the packet load = equal cost load-balancing or equal-cost multi-path (ECMP)!

Let’s check out the topology:

The summary would be:

  Codes:  L - local, C - connected, s - static, R - RIP, M - mobile, B - BGP
          D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
          N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
          E1 - OSPF external type 1, E2 - OSPF external type 2
          i - IS-IS, su - IS-IS summary, L1 - IS-IS level 1, L2 - IS-IS level 2
          ia - IS-IS inter area, * - candidate default, U - per-user static route
          o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
          a - application route
          + - replicated route, % - next hop override, p - overrides from pfR

  Gateway of last resort is not set

  R       10.0.0.0/0 [120/1] via 192.168.13.3, 00:01:16, GigabitEthernet0/2
                  [120/1] via 192.168.12.2, 00:00:24, GigabitEthernet0/0
          192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
  C           192.168.12.0/24 is directly connected, GigabitEthernet0/2
  L           192.168.12.1/32 is directly connected, GigabitEthernet0/2

          192.168.13.0/24 is variably subnetted, 2 subnets, 2 masks
  C           192.168.13.0/24 is directly connected, GigabitEthernet0/2
  L           192.168.13.1/24 is directly connected, GigabitEthernet0/2

As you can see, both R are being used here! So both routes are sharing the packet flow load!

10.3 OSPF = Open Shortest Path First

Background:
- OSPF = a link state dynamic routing protocol developed by IETF (i.e. the Internet Engineering Task Force).
- OSPF supports VLSM

10.3.1 Router ID

Router ID = identify the source of routing protocol’s packets of who sent it
How does Router ID get chosen in Cisco Routers

Picking the Router ID:

Step 1: sh run to check if there’s any router-id pre-assigned value in the run config
Step 2: Router ID can be manually configured with router-id <value>. where value is random, but must be unique.
- example:
- ```
  conf t
  hostname Router1
  ospf 1
  router-id 1.1.1.1
  end
```
- 1.1.1.1 = this ip is the router ID, and it doesn’t even need to be ping-able

Step 3: If router ID didn’t exist in run config, then highest IP among all active loopback interfaces is elected as the Router ID

loopback interface = virtual interfaces that are created on the router (e.g. int lo0 & int lo1)
Typically, loopback interface are more stable than physical interfaces

For example in the summary table…

  conf t
                
  int lo1
  ip address 172.255.255.254 255.255.255.255

  int lo2
  ip address 192.0.0.1 255.255.255.255
                
  end

to show all ip, use sh ip int brief !

  Interface               IP-address          OK?     Method      Status                  Protocol
  GigabitEthernet0/0      192.168.1.1         YES     manual      up                      up
  GigabitEthernet0/1      10.0.0.1            YES     manual      up                      up
  GigabitEthernet0/2      unassigned          YES     NVRAM       administratively down   up
  GigabitEthernet0/3      unassigned          YES     NVRAM       administratively down   up
  Loopback1               172.255.255.254     YES     manual      up                      up
  Loopback2               192.0.0.1           YES     manual      up                      up

Step 4: Let’s sort out the highest IP (the non-loopback interfaces)

Note: please drop the loopback, since we’re interested in active physical interfaces!

  Interface               IP-address          OK?     Method      Status                  Protocol
  GigabitEthernet0/0      192.168.1.1         YES     manual      up                      up
  GigabitEthernet0/1      10.0.0.1            YES     manual      up                      up
  GigabitEthernet0/2      unassigned          YES     NVRAM       administratively down   up
  GigabitEthernet0/3      unassigned          YES     NVRAM       administratively down   up

10.3.2 OSPF AD & Metric (i.e. Cost)

Background:
- Accorinding to 10.2.1, the OSPF AD is 110
- OSPF Metric = Cost, which is calculated based on bandwiths
Exercise: Finding the OSPF Cost! :D
- Topology:
- 3 steps to find the OSPF cost !
  - Step 1: Find all outgoing interfaces (i.e only factoring in s0/0/0 & g0/1). Do not calculate g0/0 please
  - Step 2: Vertify the bandwith values with sh int s0/0/0 & sh int g0/0.
    - We’re interested in the BW value!
    - Summary of sh int s0/0/0 is the following :D.
    - Serial0/0/0 is up, line protocol is up Hardware is GT98K Serial Internet Access is 192.168.1.1/24 MTU 1500 Bytes, BW 1544 Kbit/sec, DLY 2000 usec, reliability 255/255, txload 1/255, rxload 1/255 <ommitted>
    - Summary of sh int g0/0 is the following :D.
    - GiagbitEthernet0/1 is up, line protocol is up Hardware is GT98K Serial Internet Access is 192.168.1.1/24 MTU 1500 Bytes, BW 1000000 Kbit/sec, DLY 10 usec, reliability 255/255, txload 1/255, rxload 1/255 <ommitted>
  - Step 3: Calculate the metric/ cost! But there’s 2 rules to follow
    - Rule 1: 1st value is truncated to the nearest integer (basically rounded down)
    - Rule 2: 2nd value min is 1 (basically min(1,x))
    - The numerator is ALWAYS 100 Mb as the reference bandwith (i.e. RoundDown(100M/x) + Min(1, 100M/y))
    - In our case:
      - x is 1.544M (= 1544Kbit/sec)
      - y is 1000M (= 1000000 Kbit/sec)
    - Hence the equation is 100M/1.544M + 100M/1000M = 64.766 + 0.1 = 64 + 1 = 65 This is the OSPF Cost
    - Note: One should consider changing the ref bandwidth to >1000M (instead of the 100Mb) to prevent all links >100M to have same cost of 1!
  - Step 4: Let’s verify with router ospf 1 to enter router interface
    - conf t router ospf 1 auto-cost reference-bandwidth ?

10.3.3 OSPF Area

Note 1: We always just focuss on area 0 (a single area) for CCNA exams
Note 2: Area = logical collection of OSPF networks, routers, links. With hundreds of routers = achieving convergence
Note 3: Convergence happens when all routers have the same LDA in DB and all routing entries are recalculated (=Full State)

Let’s dig deep into the ospf DB sh ip ospf database to display OSPF link state DB

Summary returned: Don’t worry too much about the details here, out syll in CCNA

              OSPF Router with ID (192.168.1.1) (Process ID 1)
                  Router Link States (Area 0)
  Link ID         ADV Router          Age         Seq#            Checksum        Link Count
  192.168.1.1     192.168.1.1         215         0x8000002       0x00491F        2
  192.168.1.2     192.168.1.2         216         0x8000002       0x009A18        2
                  Net Link States (Area 0)
  Link ID         ADV Router          Age         Seq#            Checksum        
  192.168.1.2     192.168.1.2         216         0x8000001       0x00A2E6        

Multi-area design (in CCNP) (aka: The hierarchical design) has some benefits: (Out syll as well)
- 1. Faster convergence
- 1. Less routing overhead (less downtime)
- 1. Limitting network instability to single area
- 1. Allows extensive control routing updates (passing from 1 area to another), non-multi area cannot filter
In non-cisco specific OSPF, area 0 = backbone area for connecting other area. In normal setting, all ABRs should have link connecting to area 0

10.3.4 OSPF Adjacency or OSPF Neighbor Relationships

3 OSPF opertrations
- 1. Neighbor Discover (Very Important)
- 1. Link-state info exchange
- 1. Best-path calculation
OSPF main neighbor tables with hello packets during router start up
- Hello packets are sent to all OSPF enabled interfaces on every 10 sec intervals
- If no hello packet is sent before dead interval is up (defualt it’s 40 sec), OSPD neiighbor is considered dead and will be removed from the neighbor list
- Set up hello interval and dead interval with these ip ospf hello-interval ? & ip ospf dead-interval ?.
  - conf t int g0/0 ip ospf hello-interval ? ip ospf dead-interval ?
Requirements of forming OSPF Adjacency, there are 4
- 1 - Same OSPF Area ID
- 2 - Same OSPF Hello Intervals
- 3 - Same MTU = Maximum Transmission Unit, largest prootocl data unit (PDU) to communicate in single network layer transactions
- 4 - Outsyll other itms to be discussed in CCNP!
Good OSPF topology design
Bad OSPF topology designs
- 1,2,3 - are the following

10.3.5 OSPF Network Types (Point to Point Broadcast) & Designated Router (DR)

Background:
- OSPF has different network types to affect adjacency and OSPF interface behavior
- CCNA will focus on 2 types of network types: p2p (point to point) & broadcast

10.3.5.1 Point to Point

Suitable Enviornment? Ans: Point to point env
No concept of BDR & DR
Default run point to point for OSPF serial interfaces
Topology:

10.3.5.2 Broadcast

Suitable Enviornment? Ans: multi-point / point to multipoint env
Will elect and pick out BDR & DR.
DR = Designated Router = Collect and redistribute LDA + other info
BDR = Backup Designated Router = act as backup of DR
Topology:

10.3.5.a How to elect DR

Highest OSPF interface priority default is 1, if set pirority = 0 then the device gives up on being DR/ BDR
Highest OSPF Router ID will be DR after priority values

Note: By default: IP 224.0.0.5 is reserved for all OSPF devices (routers), excluding Computer Note: By default: IP 224.0.0.6 is reserved for all OSPF DR and BDR Devices

10.3.5.b Configuring interface priority

```
conf t
int g0/0
ip ospf priority ?
```
OSPF routers form full adjacency (can exchange complete routing info) to DR & BDR.
ADROTHER router form 2-way adjacency (can exchange complete routing information) w/ other DROTHER router.
Topology:
Remark:
- 224.0.0.5 - Multicast address, Class D is reserved for all OSPF device
- 244.0.0.6 - Multicast address, Class D is reserved for all OSPF DR device

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