MPLS + BGP advanced features

Source: Internet
Author: User

MPLS + BGP advanced features
Will LDP allocate tags for BGP routes? Why? --- Previously we mentioned that LDP assigns tags to IGP and never assigns tags to BGP. the following uses an experiment to illustrate how LDP + BGP works. forwarding method of a pure IP Network: In this topology, R1 and R4 establish an EBGP neighbor. r3 and R5 establish EBGP neighbors. build an IBGP neighbor between R1 and R3. IGP protocol between R1-R2-R3 is OSPF. network mask protocol 255.255.255r5 bgp network mask protocol 255.255.255 in R4 bgp: Eventually, both R4 and R5 will learn the peer BGP Route through BGP. first, there is a problem: R4 ping source loo0, or R5 ping sou loo0. Will it pass? ------------- The answer is no. by doing experiments, we can prove that. the reason is above R2. if it is a pure IP address, R2 is a routing black hole. on R2, neither nor is available. if you want to connect the two vrouters R4 and R5, there are many methods unless R2 is also involved in BGP. R2 acts as the RR route reflector. R1 and R3 are both R2 clients. or use the consortium. in short, R2 must be able to learn the routes related to, so that communication can be performed in a pure IP environment. otherwise, R2 is a routing black hole. MPLS label forwarding method: the topology is still displayed, but the mpls ip is run between R1, R2, and R3 in the middle AS 100 region. at this time, R2 still does not run BGP. other conditions are the same as those for the above pure IP forwarding. in this case, R4 network enters BGP. r5 network enters bgp. q: Can I ping R5 on R4? Why? ------- The experiment showed that it could pass. Conclusion: an IP packet is received on the ASBR. For R1, the source is the purpose is, which is to import the e0/1 IP address of R1 from e0/0 MPLS of R1. the destination is to a bgp entry. at this time, a label will be added, and a label will be added to the next hop of the bgp Route. this is why R2 does not go to route or, but there are still labels because the labels are for or R2 has a route prefix and associated tag ing. Therefore, make sure that the ASBR has a detailed route. Otherwise, packet loss will occur because there are no entries in the core route table in the ASBR. next let's take a look at how this data stream goes: On R4, ping source loo0.1. First, R4 needs the prefix routing information of here, in the R4 core route table, the next hop for this route is accessible. then the data packet is sent to R1. 2. When the IP packet is sent to R1, It is IP ---> MPLS. Check the CEF table. the following are all related table items of R1. first, R1 has a route entry with the core route table: show ip routeB [200/0] via, 02:40:13 here, R1 is going to, And the next hop of the bgp Route Is secondly, in the show ip cef table, we found that the prefix of was assigned a label = 17 by remote R2. again, in LFIB, the label 17 is actually allocated to (which is the next hop with the bgp prefix ).. Finally, the packet is pushed to a label of 17 and forwarded to R2.3 through the e0/0 of R1. When the MPLS packet carries the label of 17, it enters the e0/0 of R2. the label forwarding table of R2 is as follows: for incoming 17 MPLS packets, this label is displayed on R2, because is a direct connection of R3, r3 will allocate an implicit null label to the prefix and distribute it to R2. when the packet arrives at R2, the label will be stripped and pop label (the penultimate pop-up Mechanism of PHP) will be removed ). then the IP packet is forwarded to R3.4. After R3 receives the packet. the message category is search route table: route table above R3: On R3, it reaches, And the next hop is a direct connection route yes. Therefore, the data packet is forwarded to R5.5, and the data packet is forwarded to R5. ========================================================== ===== Thinking: if R1 and R3 use the physical interface instead of loopback as the bgp source, can R4 and R5 still communicate? The previous application was the IBGP established by R1 and R3 using the loopback port. Think, if you use a direct connection to build an IBGP neighbor, can R4 and R5 still communicate through the MPLS domain? Why? ----------- The answer is no. Cause: After the data packet arrives at R1, the route table on R1 will become: show ip routeB [200/0] via, R2 will think that is its own direct connection route, then R2 will assign an empty tag to the labels for are not distributed to R1 and R3.R1. after the IP packet arrives at R2, R2 queries the route table without tags. Because R2 does not enable bgp at all, and there is no route for, so R2 becomes a routing black hole. the main purpose is to use tags to send data packets to R3, so that R3 can query and forward data through routing.

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