CEF and host route

[verma-rohit]

Hi,

R1-------------R2

R1:

lo0: 1.1.1.1/32

R2:

lo02.2.2.2/32

I have MP-BGP configured between R1 and R2.

Everything works fine, however on R1 i summarized loopback in ospf as 1.1.0.0/16. R2 routing table has 1.1.1.1/32 route  and has 1.1.0.0/16 as configured. However i noticed CEF did install 1.1.1.1/32 host route which i find strange and I believe it has to do with MP-BGP configuration.

When i remove MP-BGP and runs plain ospf between two routers, host route 1.1.1.1/32 no longer exits. So why Mp-BGP is injecting a host route  when route is summarized?

R2: When MP-BGP is in operation

R2#show ip cef
Prefix Next Hop Interface
0.0.0.0/0 no route
0.0.0.0/8 drop
0.0.0.0/32 receive
1.1.0.0/16 192.168.1.1 Ethernet0/0
1.1.1.1/32 192.168.1.1 Ethernet0/0
2.2.2.2/32 receive Loopback0
127.0.0.0/8 drop
192.168.1.0/24 attached Ethernet0/0
192.168.1.0/32 receive Ethernet0/0
192.168.1.1/32 attached Ethernet0/0
192.168.1.2/32 receive Ethernet0/0
192.168.1.255/32 receive Ethernet0/0
224.0.0.0/4 drop
224.0.0.0/24 receive
240.0.0.0/4 drop
255.255.255.255/32 receive

R2: When MP-BGP is removed      

R2#show ip cef
Prefix Next Hop Interface
0.0.0.0/0 no route
0.0.0.0/8 drop
0.0.0.0/32 receive
1.1.0.0/16 192.168.1.1 Ethernet0/0
2.2.2.2/32 receive Loopback0
127.0.0.0/8 drop
192.168.1.0/24 attached Ethernet0/0
192.168.1.0/32 receive Ethernet0/0
192.168.1.1/32 attached Ethernet0/0
192.168.1.2/32 receive Ethernet0/0
192.168.1.255/32 receive Ethernet0/0
224.0.0.0/4 drop
224.0.0.0/24 receive
240.0.0.0/4 drop
255.255.255.255/32 receive

[Harold Ritter]

> I understand now that label swap from R2 to R1 for R1 lo0 is for network 1.1.0.0/16 but what is bothering me that next hop is > 1.1.1.1 and mpls forwarding table for the said next hop ( 1.1.1.1) shows no label.

Bear in mind that on the ingress PE, it is the FIB that is referenced, not the LFIB. So it doesn't matter if there is not entry for 1.1.1.1/32 in the LFIB as such.

Regards,

[verma-rohit]

 Isn't R1  Injecting the route and it is the ingress PE ? Remote   router is R2, so it looks into mpls forwarding table for next hop of 1.1.1.1 which yields no label. 

[Harold Ritter]

The ingress PE refers to the node where traffic originates. R2 (the ingress PE for traffic towards R1) performs the lookup against the FIB for IP to MPLS traffic. The P (MPLS to MPLS) and egress PE (MPLS to IP) perform the lookup against the LFIB.

Regards,

[verma-rohit]

Thank you so much Harold and i i promise it would be my last post.

PE1.-----PE 2( also servers as reflector, NOT P)-------PE3 

1. PE 3 inject routes and I  issue ping from PE 1

2. Ingress router PE1 does next hop look up on FIB and goes to PE2.

3. PE 2 check LFIB and forward it to P3.

4. ICMP reply, egress router PE 3, does LFIB lookup  and follow along to P1

So, on ingress its FIB lookup and LFIB lookup on engress.

Secondly does IOS and NXOS follow the traditional MP BGP forwarding rule ? I came across situation in production where :

PE 1(IOS) .....PE 2 (NX OS) , PE 2 was injecting routes but a summarization on PE 2 , Where its loopback 0 got summarized, broke the data plane and there was a a outage reported.