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Chapter 12: Troubleshooting Basic IPv4/IPv6 Routing and GRE Tunnels

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Layer 3 Packet-Forwarding Process Step 1.
Step 1.
PC1 compares its IP address and subnet mask of 192.168.1.2/24 with the destination IP address 192.168.3.2
Layer 3 Packet-Forwarding Process Step 2.
Step 2.
Router R1 receives the frame sent from PC1, and because the destination MAC is R1's, R1 tears off the Layer 2 header and interrogates the IP (Layer 3) header. An IP header contains a time-to-live (TTL) field, which is decremented once for each router hop. Therefore, router R1 decrements the packet's TTL field. If the value in the TTL field is reduced to zero, the router discards the packet and sends a time-exceeded Internet Control Message Protocol (ICMP) message back to the source.
IP routing table
When a router needed to route an IP packet, it consulted its IP routing table to find the best match. The best match is the route that has the longest prefix.
Layer 3 Packet-Forwarding Process Step 3.
When router R2 receives the frame, it removes the PPP header, and then dec- rements the TTL in the IP header, just as router R1 did. Again, assuming the TTL did not get decremented to zero, router R2 interrogates the IP header to determine the destination network. In this case, the destination network of 192.168.3.0/24 is directly attached to router R2's Fast Ethernet 0/0 interface. Similar to how PC1 sent out an ARP request to determine the MAC address
What is (FIB)
Forwarding Information Base
Layer 3 to Layer 2 mapping table
In the previous figure, router R2's A-R-P cache contained Layer 3 to Layer 2 mapping information. Specifically, the ARP cache had a mapping that said a MAC address of (2222.2222.2222) corresponded to an IP address of 192.168.3.2. An ARP cache is the Layer 3 to Layer 2 mapping data structure used for Ethernet-based networks,
Forwarding Information Base
The FIB contains Layer 3 information, similar to the information found in an IP routing table. In addition, an FIB contains informa- tion about multicast routes and directly connected hosts.
Adjacency table
When a router is performing a route lookup using CEF, the FIB references an entry in the adjacency table. The adjacency table entry contains the frame header information required by the router to properly form a frame. Therefore, an egress interface and a next-hop MAC address would be in an adjacency entry for a multipoint interface, whereas a point-to-point interface would require only egress interface information.
Default Administrative Distance of Route Sources Connected interface
0
Troubleshooting the Packet-Forwarding Process
Router's IP routing table.
CEF is operating in the data plane and uses the FIB
Default Administrative Distance of Route Sources Static route
1
Default Administrative Distance of Route Sources EIGRP summary route
5
Default Administrative Distance of Route Sources eBGP
20
Default Administrative Distance of Route Sources EIGRP (internal)
90
Default Administrative Distance of Route Sources OSPF
110
Default Administrative Distance of Route Sources IS-IS
115
Default Administrative Distance of Route Sources RIP
120
Default Administrative Distance of Route Sources EGP
140
Default Administrative Distance of Route Sources ORD
160
Default Administrative Distance of Route Sources EIGRP (external)
170
Default Administrative Distance of Route Sources iBGP
200
Default Administrative Distance of Route Sources Unknown (not believable)
255
Verifying the Administrative Distance of a Route in the Routing Table
Show Ip route 10.1.1.0
Static routes are manually configured
By default are the second most trustworthy sources of routing information, with an AD of 1.
Configuring a Static Route on R1 with Next-Hop Option
R1(config)#ip route 10.1.3.0 255.255.255.0 10.1.12.2 8
Verifying a Static Route on R1
R1#show ip route static
MAC Address Lookup in ARP Cache
R1#show ip arp
To create an IPv6 static route, you use the
R1(config)#ipv6 route 2001:DB8:0:3::/64 gigabitEthernet 1/0 FE80::2 8
Are there Broadcasts with IPv6.
No
IPv6 does not use ARP.
It uses ?
NDP (Neighbor Discovery Protocol), which is multicast based
Viewing the IPv6 Neighbor Table on R1
R1#show ipv6 neighbors
When R1 receives a packet destined for 2001:db8:0:3::3, it determines based on the static route that it is directly connected to Gig1/0 (which it is not according to the figure). Therefore, R1 sends an
N-S out Gig1/0 for the MAC address associated with (2001:db8:0:3::3) using the solicited-node multicast address (FF02::1:FF00:3.)
What is GRE ?
Generic routing encapsulation (GRE) is a tunneling protocol that is used to encapsulate various types of network layer packets inside a transport protocol (GRE) so that they can be transported over an IP network.
GRE Encapsulated Packet
When troubleshooting GRE issues, you need to consider the following
Are the remote devices reachable across the public network?
Are the tunnel IP addresses in the same subnet?
Are the correct tunnel source and destination IP addresses specified?
Is the correct tunnel mode specified?
Is an access control list (ACL) blocking GRE packets?
Is fragmentation occurring due to insufficient maximum transmission unit (MTU)? Because the GRE header is 24 bytes,
Is the recursive routing table lookup pointing back to the tunnel?
Is the routing protocol enabled on the tunnel interface?
When using IPsec with GRE
GRE encapsulates the original packet payload first, and then encryption occurs next with IPsec to protect the GRE packet.
Two different IPsec modes exist that you can use to encapsulate the GRE packet
IPsec tunnel mode will encapsulate and encrypt the entire GRE packet, including the Transport Protocol header. Because the Transport Protocol header is being encapsulated and encrypted, IPsec has to include a new IP header. IPsec transport mode will only encapsulate and encrypt the carrier protocol and the passenger protocol. Therefore, the Transport Protocol header can be reused by IPsec and reduce overhead.
Benefits of using GRE and IPsec for site-to-site VPNs include the following:
■ In addition to supporting IPv4 as the passenger protocol, it provides support for other Layer 3 protocols.
■ It provides support for multicast and routing traffic across the IPsec VPN.
■ With a hub-and-spoke topology, it reduces the management overhead needed to maintain IPsec tunnels because a minimum number of tunnels is used to provide full connectivity.
Displays only the static routes in a routers routing table.
show ip route static
Displays a router's best route to the specified IP address.
show ip route (ip_address)
Displays information (for example, next-hop IP address and egress interface) required to forward a packet, similar to the output of the show ip route ip_address command. (The output of this command comes from CEF. Therefore, routing protocol information is not presented in the output.)
show ip cef (ip_address)
Displays a router's best route to the specified network if the specific route (with a matching subnet mask length) is found in the router's IP routing table.
show ip route (ip_address subnet_ mask)
Displays all routes in a router's IP routing table that are encompassed by the specified network address and subnet mask.
show ip route (ip_address subnet_ mask longer- prefixes)
Displays the static IPv6 routes configured on a device.
show ipv6 route static
Displays information from a router's FIB showing the information needed to route a packet to the specified network with the specified subnet mask.
show ip cef (ip_address subnet_ mask)
Displays Frame Relay PVC DLCIs associated with next- hop IP addresses.
show frame-relay map
Displays the Layer 3 IPv6 address to Layer 2 MAC address mappings.
Show ipv6 neighbors
Displays a router's ARP cache, containing IPv4 address to MAC address mappings.
Show I-P A-R-P
Displays whether Proxy ARP is enabled on an interface as well as the IPv4 address and mask assigned to the interface.
show IP interface Fast 0/0
Displays the Layer 2 frame header information in a router's CEF adjacency table that is used to encapsulate a frame being sent to an adjacent router.
show adjacency detail
Displays the configuration of a tunnel interface in the running configuration.
show run interface tunnel 4
Displays the status of a tunnel, the IP address of the tunnel, the tunnel source and destination, along with the tunnel mode and the tunnel transport MTU.
Show interfaces tunnel tunnel_ number
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