You have a router running BGP for the MPLS network and OSPF for the local LAN network at the
sales office. A route is being learned from the MPLS network that also exists on the OSPF local
network. It is important that the router chooses the local LAN route being learned from the
downstream switch running OSPF rather than the upstream BGP neighbor. Also, if the local OSPF
route goes away, the BGP route needs to be used. What should be configured to make sure that
the router will choose the LAN network as the preferred path?
A.
Static route needs to be added
B.
Floating static route needs to be added
C.
Bgp backdoor command
D.
Ospf backdoor command
Explanation:
We often run into situations where we have two different routes to a network and we know one is
faster than the other. However, a router will only keep one route in the routing table and that will
be the one with the shortest administrative distance. Now if two routers (R1 and R2) in an AS are
directly connected to each other via a 10meg ethernet running EIGRP (Admin distance 90) but
they also have a T1 connection to a service provider running EBGP (Admin distance 20). Now, as
human beings, we are smart enough to see that if we are R1 and have to reach a network located
behind R2, we should use the EIGRP route via 10meg connection.
Now think of this from a routers perspective, R1 sees two routes to that network, via EIGRP and
EBGP. R1 is going to take the slower path via EBGP because EBGPs admin distance is 20.
Changing the default administrative distances is not recommended because that may lead to
routing loops. So this is where we can use the network backdoor command so router will prefer
EIGRP route over EBGP route. BGP treats the network specified by the network backdoor
command as a locally assigned network but it doesn’t advertise the network in BGP updates. In
short, BGP network backdoor command changes the admin distance of that network to 200 thus
making it a worst route compared to EIGRP (90). I think this is a great feature that I never utilized
before. This actually makes sense and is a better way of doing things than changing the admin
distance. Here are a few steps to accomplish this task:
•enable
•configure terminal•router bgp autonomous-system-number
•neighbor ip-address remote-as autonomous-system-number
•network ip-address backdoor
•end
Example with the backdoor command being used:
R2#sh ip route
Codes: 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
Gateway of last resort is not set
B 1.0.0.0/8 [20/0] via 10.1.12.1, 00:04:42
C 2.0.0.0/8 is directly connected, Loopback0
B 3.0.0.0/8 [20/0] via 10.1.12.1, 00:00:48
10.0.0.0/24 is subnetted, 2 subnets
R 10.1.13.0 [120/1] via 10.1.12.1, 00:00:13, Serial0/0.21 C 10.1.12.0 is directly connected,
Serial0/0.21
150.1.0.0/24 is subnetted, 3 subnets
C 150.1.23.0 is directly connected, FastEthernet0/0
D 150.1.3.0 [90/156160] via 150.1.23.3, 00:00:07, FastEthernet0/0 C 150.1.2.0 is directly
connected,
Loopback1
Example with link between two routers down:
R2#sh ip route
Codes: 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
Gateway of last resort is not set
B 1.0.0.0/8 [20/0] via 10.1.12.1, 00:20:41
C 2.0.0.0/8 is directly connected, Loopback0
B 3.0.0.0/8 [20/0] via 10.1.12.1, 00:16:46
10.0.0.0/24 is subnetted, 2 subnetsR 10.1.13.0 [120/1] via 10.1.12.1, 00:00:04, Serial0/0.21 C 10.1.12.0 is directly connected,
Serial0/0.21
150.1.0.0/24 is subnetted, 3 subnets
C 150.1.23.0 is directly connected, FastEthernet0/0
B 150.1.3.0 [200/0] via 10.1.12.1, 00:00:16
C 150.1.2.0 is directly connected, Loopback1
Note: Admin distance of 20 for the eBGP route.
Conditional BGP Route Advertisement:
BGP has a neat feature where you can control what routes to advertise to a certain neighbor. So
lets say you are in the middle of two ASs and are passing routes between them. You can use
conditional advertisement to say if a network connected to you is down; don’t advertise certain
network coming in from the other router. It basically revolves around the command advertise-map.
Cisco has some nice summary steps to explain the process:
1. Enable
2. configure terminal
3. router bgp autonomous-system-number
4. neighbor {ip-address | peer-group-name} remote-as autonomous-system-number
5. neighbor ip-address advertise-map map-name {exist-map map-name | non-exist-map mapname}
6. exit
7. route-map map-tag [permit | deny] [sequence-number]
8. Match ip address {access-list-number [access-list-number… | access-list-name…] | access-listname [access-list-number… | access-list-name] | prefix-list prefix-list-name [prefix-list-name…]}
9. Repeat Steps 7 and 8 for every prefix to be tracked.
10. Exit
11. access-list access-list-number {deny | permit} source [source-wildcard] [log]
12. Repeat Step 11 for every access list to be created.
13. exit
The route map associated with the exist map or nonexist map specifies the prefix that the BGP
speaker will track. The route map associated with the advertise map specifies the prefix that will
be advertised to the specified neighbor when the condition is met.