Routing Concepts

Question 1

IP Routing Process

Answer 1

Packet routing involves three main steps:

1. Routing
   
   1. Find the outgoing interface for the packet
2. Switching
   
   1. Move the packet between interfaces
3. Encapsulation
   
   1. Rebuild the packet’s layer 2 header

Question 2

Step 1 - routing process

Find the outgoing interface

Answer 2

What does the router do when a packet enters?

• Find the longest match to the destination
  
  • show ip route 1.2.3.4
    
    • 1.0.0.0/8
    • 1.2.0.0/16
    • 1.2.3.0/24
• Recurse to the outgoing interface
  
  • 1.2.3.4 via 5.6.7.8
  • 5.6.7.8 via 9.0.1.2
  • 9.0.1.2 via 3.4.5.6
  • 3.4.5.6 directly connected, GigabitEthernet1
• Note: a route cannot be installed in the routing table unless the recursive lookup is successful.

Question 3

Metric vs Distance

Answer 3

What if there are multiple longest matches?

• Same protocol
  
  • Choose lowest metric
  • Rules vary per protocol
    
    • E.g. OSPF internal over external
• Different protocols
  
  • Choose lowest Administrative Distance

Question 4

Step 2 - switching process

Move the packet between interfaces

Answer 4

• Move the packet between interfaces
  
  • Called the “switching path”
  • Process, Fast, CEF, etc.
• This is where load balancing occurs
  
  • Ideally per flow, not per packet
• Verification
  
  • show ip interface
  • show ip cef [address] [detail]
  • show ip cef exact-route [source] [destination]

Question 5

Step 3 - encapsulation process

Build the Layer 2 Header

Answer 5

• Build the layer 2 header
  
  • Header type depends on egress interface
    
    • E.g. Ethernet vs. PPP
• For multipoint interfaces, layer 3 to layer 2 resolution is required
  
  • Ethernet
  • Frame Relay Multipoint
  • ATM Multipoint
  • mGRE (DMVPN)

Question 6

Encapsulation on Ethernet

Resolve the next-hop IP to next-hop MAC

Answer 6

• Resolve the next-hop IP to next-hop MAC
  
  • Address Resolution Protocol (ARP)
• Check the ARP cache first
  
  • show ip arp
• If no match, broadcast the request
  
  • debug arp

Question 7

Encapsulation on mGRE (DMVPN)

Resolve the tunnel IP to the NBMA IP

Answer 7

• Resolve the tunnel IP to the NBMA IP
  
  • Next-Hop Resolution Protocol (NHRP)
• Check the NHRP cache first
  
  • show ip nhrp
• If no match, ask the next-hop server (NHS)
  
  • debug nhrp
  • debug dmvpn

Question 8

Routing to a Next-Hop

If multipoint (Ethernet), find MAC of next-hop

Answer 8

• Recurse to the interface
• If multipoint, resolve next-hop
• Example
  
  • ip route 10.0.0.0 255.0.0.0 1.2.3.4
  • Find interface for 1.2.3.4
  • If multipoint, find layer 2 address of 1.2.3.4

Question 9

Routing to a Multipoint Interface

Answer 9

• Recursion not required
• Resolve layer 2 address for final destination
  
  • Ethernet Proxy-ARP
  • NHRP mappings
• Example
  
  • ip route 10.0.0.0 255.0.0.0 FastEthernet0/0
  • Find layer 2 address of 10.0.0.1

Question 10

Routing to a Point-to-Point Interface

Answer 10

• Recursion not required
• Layer 2 resolution not required
• Example
  
  • ip route 10.0.0.0 255.0.0.0 Serial0/0.1

Question 11

Default Routing

Answer 11

• To a next-hop
  
  • Use the layer 2 address of the next hop for all layer 3 destinations
• To a multipoint interface
  
  • All destinations require layer 3 to layer 2 resolution
  • Layer 2 mapping table size problems
• To a point-to-point interface
  
  • No layer 3 to layer 2 resolution required

Question 12

Other Default Routing

Answer 12

• ip default-gateway
  
  • Only when IP routing is off
• ip default-network
  
  • Network flagged as default in routing advertisements
  • Must be a classful network that is not directly connected

Question 13

What is Reliable Static Routing?

Answer 13

• Only criteria for installing a static route is the route recursion process
  
  • Can I find the outgoing interface?
• Static routes don’t know the network conditions on the other end of the link
  
  • Next hop may be reachable or may be down
• Reliable static routes combine static routing with enhanced objects
  
  • Enhanced object calls IP SLA feature to track remote reachability
  • If tracked object is down, static route is removed from the routing table
• Allows for more intelligence with floating static routes
  
  • Primary static route tracks remote next hop
  • If remote next hop is unreachable, remove the static route
  • Floating static replaces it and connectivity can be maintained

Question 14

Configuring Reliable Static Routing

Answer 14

• Define IP SLA agreement
  
  • E.g. ping the remote end of the link
• Define tracked object
  
  • Tracked object calls IP SLA agreement
• Tie tracked object to static route

Question 15

What Is Policy Based Routing (PBR)?

Normal IP routing is destination based.

Answer 15

• Normal IP routing is destination based
  
  • Find the longest match to the destination
  • Route the packet towards the next-hop
• PBR allows forwarding decision to be based on…
  
  • Source
  • Destination
  • Protocol type (FTP, HTTP, etc.)
  • Incoming interface

Question 16

How Policy Based Routing Works

Answer 16

• Traffic criteria defined by a route-map match
  
  • Permit means to policy route
  • Deny means to use normal forwarding
  • match [interface | ip address | …]
• Action defined by route-map set
  
  • set ip [default] next-hop
  • set [default] interface
• Policy can be verified with…
  
  • debug ip policy

Question 17

Policy Routing Variations

Answer 17

PBR can apply to two types of traffic:

• Incoming traffic
  
  • PBR applied at the link level
  • ip policy route-map [route-map]
• Locally originated traffic
  
  • PBR applied globally
  • ip local policy route-map [route-map]
  • Be careful, some versions don’t have control plane checks

Question 18

Policy Routing Caveats

Answer 18

• Most platforms can’t hardware accelerate PBR
• Platforms that do have strict criteria limits
• Outside of this PBR should be used sparingly
  
  • High CPU due to interrupts means process switching problem